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Science: Why female moths find toxic males a turn-on

By DAVID BRADLEY

A species of moth found in the southern US produces a sex attractant
with a difference. The compound, generated by males, tells a female just
how much of a second, defensive, chemical a male is likely to pass on to
her in his sexual fluid if she decides to mate with him.

Chemists Jerrold Meinwald and the late Carl Harris of Cornell University,
New York, studied the arctiid moth (Utetheisa ornatri). They discovered
that it makes a sex pheromone called hydroxydanaidal from monocrotaline,
a chemical in the plants it feeds on (see diagram). Monocrotaline is a so-called
pyrrolizidine alkaloid, one of a class of bitter-tasting, often highly toxic,
compounds which many plants make.

Male moths exude hydroxydanaidal from two brush-like antennas, known
as coremata, located on their heads. Females of the species can detect very
small amounts of the pheromone by means of two antennae which act as chemical
receptors.

Meinwald and Harris discovered that female moths are always attracted
to the males which emit the most hydroxydanaidal. They also discovered that
the amount of pheromone which a male emits is directly correlated with the
male’s supply of the monocrotaline alkaloid (Proceedings of the National
Academy of Sciences, vol 88, p 9224).

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David Dussourd and Thomas Eisner, biologists at Cornell, believe that
females can assess the worth of a potential mate from the amount of pheromone
that a male gives off. The males which produce the strongest pheromone signal
have the greatest supply of alkaloid. By mating with males that are well-endowed
with the pheromone, female moths ensure that they will acquire a large ‘nuptial
gift’ of alkaloid with the sperm-containing fluid or spermatophore.

Receiving such a gift has several benefits, say Dussourd and Eisner.
When the female moth lays her eggs some of the alkaloid will be transferred
to them, providing the growing larvae with a chemical defence with which
to deter predators, such as birds.

Another benefit of mating with alkaloid-rich males is that it is safer
for the female. Mating sometimes lasts more than eight hours, during which
the female is immobile and vulnerable to predators. But if a male carries
a large amount of the toxic alkaloid, the female will also benefit from
his chemical defences.

The moths obtain the alkaloid, monocrotaline, from the seeds of the
plant, Crotalaria spectabilis. These are the staple diet of the moths during
their larval stage. According to the researchers, another reason why females
choose males with high pheromone levels is that these are the ones who got
the lion’s share of the seeds when they were larvae. Any offspring of such
aggressively voracious males are likely to be just as aggressive and competitive.